Truncate GSO when necessary to avoid excessive padding

quinn-proto/src/connection/mod.rs

@@ -683,6 +683,24 @@ impl Connection {
                     }
 
                     if num_datagrams > 1 {
+                        // If too many padding bytes would be required to continue the GSO batch
+                        // after this packet, end the GSO batch here. Ensures that fixed-size frames
+                        // with heterogeneous sizes (e.g. application datagrams) won't inadvertently
+                        // waste large amounts of bandwidth. The exact threshold is a bit arbitrary
+                        // and might benefit from further tuning, though there's no universally
+                        // optimal value.
+                        const MAX_PADDING: usize = 16;
+                        let packet_len_unpadded = cmp::max(builder.min_size, buf.len())
+                            - datagram_start
+                            + builder.tag_len;
+                        if packet_len_unpadded + MAX_PADDING < segment_size {
+                            trace!(
+                                "GSO truncated by demand for {} padding bytes",
+                                segment_size - packet_len_unpadded
+                            );
+                            break;
+                        }
+
                         // Pad the current packet to GSO segment size so it can be included in the
                         // GSO batch.
                         builder.pad_to(segment_size as u16);

quinn-proto/src/tests/mod.rs

@@ -3042,3 +3042,25 @@ fn datagram_gso() {
         ((29 + DATAGRAM_LEN) * DATAGRAMS) as u64
     );
 }
+
+#[test]
+fn gso_truncation() {
+    let _guard = subscribe();
+    let mut pair = Pair::default();
+    let (client_ch, _) = pair.connect();
+
+    let initial_ios = pair.client_conn_mut(client_ch).stats().udp_tx.ios;
+
+    // Send three application datagrams such that each is large to be combined with another in a
+    // single MTU, and the second datagram would require an unreasonably large amount of padding to
+    // produce a QUIC packet of the same length as the first.
+    info!("sending");
+    for len in [1024, 768, 768] {
+        pair.client_datagrams(client_ch)
+            .send(vec![0; len].into(), false)
+            .unwrap();
+    }
+    pair.drive();
+    let final_ios = pair.client_conn_mut(client_ch).stats().udp_tx.ios;
+    assert_eq!(final_ios - initial_ios, 2);
+}